Mercedes-Benz 2013 C Class Coupe - Manual (Page 312)

Retrofitted anti-glare film
Retrofitted anti-glare film on the inside of the
windows can interfere with radio/mobile
telephone reception. This is particularly the
case for conductive or metallic-coated films.
You can obtain information about anti-glare
film from a qualified specialist workshop.
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Features
Stowing and features